The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Many power supplies use fast switching diodes to rectify the output voltage of the power supply. In general, diodes allow current to flow in one direction through the diode when a forward bias voltage is applied across the diode. When a reverse bias voltage is applied across the diode, the diode should act as an open switch and prevent current from flowing in the opposite direction.
FIG. 1 illustrates a conventional boost converter, which is indicated generally by reference numeral 100. The converter 100 includes an inductor 102, a diode 104, a switch 106 and an output capacitor 108. The operation of a conventional boost converter is well known to those skilled in the art and will therefore be explained only briefly herein. In use, a DC input voltage 110 is applied between the inductor 102 and a reference node 112 (also referred to as ground). The switch 106 opens and closes at a high frequency. When the switch 106 is closed, current flows from the DC input voltage 110, through the inductor 102 and the switch 106, to ground 112. When the switch 106 opens, current flows through the inductor 102 and the diode 104 to charge the output capacitor 108 and supply power to a load 114. When the switch 106 closes again, the capacitor 108 discharges current to the load 114. During this time, the diode 104 should prevent current from flowing back toward the switch 106.
However, the high switching frequency of modern power supplies can result in a large change in voltage across the diode 104 as a function of time (dv/dt). This leads to a brief time during which a reverse current flows through the diode 104 (from the cathode to the anode). This reverse current is known in the art as diode reverse recovery current.
FIG. 2 illustrates the current 216 flowing through the diode 104 before and after the switch position changes from open to closed at time t0. As shown in FIG. 2, the current decreases to zero amps at time t1. At that point, the current becomes negative and begins to flow in the opposite direction, resulting in a reverse recovery current that peaks at time t2. The reverse recovery current then decreases back to zero amps at time t3.
The reverse recovery current increases the electromagnetic interference (EMI) generated by the converter 100. The reverse recovery spike illustrated in FIG. 2 also limits the switching frequency of the converter 100 and requires use of a more robust and hence more expensive diode 104.
FIG. 3 illustrates a boost converter 300 employing one known method for suppressing diode reverse recovery current. The boost converter 300 includes a main winding 302, a main diode 304, a switch 306, and an output capacitor 308. An input voltage 310 is coupled between the main winding and ground 312 so that an output voltage can be supplied to a load 314. The boost converter 300 includes additional elements, generally indicated by reference number 318, to limit diode reverse recovery current. Specifically, the additional elements 318 include an auxiliary winding 320 (coupled to the main winding 302) connected in series with an inductor 322 and an auxiliary diode 324.
When the switch 306 is closed, current flows through the main winding 302 and the switch 306 to ground 312. When the switch 306 opens, current initially flows through the main winding 302 and the main diode 304 to the output capacitor 308 and load 314. This results in a voltage across the additional elements 318 that forward biases the auxiliary diode 324 and causes current to flow through the additional elements 318. This current increases until all current from the main winding 302 is channeled through the additional elements 318. This diversion of current from the main diode 304 to the auxiliary diode 324 is completed before the switch 306 closes to begin the next cycle. Therefore, when the switch 306 closes, no current is flowing through the main diode 304 and the reverse recovery current in the main diode 304 is greatly decreased. Additionally, the leakage inductance of the coupled main and auxiliary windings 302, 320 prevent the current flowing through the additional elements 318 from changing rapidly. As a result, the reverse recovery current through the auxiliary diode 324 is limited.
FIG. 4 illustrates a boost converter 400 employing another known method for suppressing diode reverse recovery current. The boost converter 400 includes a main winding 402, a main diode 404, a switch 406, and an output capacitor 408. An input voltage 410 is coupled between the main winding 402 and ground 412 so that an output voltage can be supplied to a load 414. The boost converter 400 also includes additional elements, generally indicated by reference number 418, to limit diode reverse recovery current. Specifically, the additional elements 418 include an auxiliary winding 420 (coupled to the main winding 402) connected in series with an auxiliary diode 424. The auxiliary winding 420 has more turns than the main winding 402. Additionally, the auxiliary winding 420 and the auxiliary diode 424 are connected to the input voltage 410 in parallel with the main winding 402 and the main diode 404.
When the switch 406 is closed, current flows through the main winding 402 and the switch 406 to ground. When the switch 406 opens, current initially flows through the main winding 402 and the main diode 404 to the output capacitor 408 and load 414. Current then begins to flow, at an increasing rate, through the additional elements 418 until all current from the voltage source 410 is channeled through the additional elements 418. This diversion of current from the main diode 404 to the auxiliary diode 424 occurs before the switch 406 closes to begin the next cycle. Therefore, when the switch closes, no current is flowing through the main diode 404 and the reverse recovery current in the main diode 404 is greatly decreased. Additionally, various factors including the leakage inductance of the coupled main and auxiliary windings 402, 420 prevent the current flowing through the additional elements 418 from changing rapidly. As a result the reverse recovery current through the auxiliary diode 424 is limited.
Although the converters 300, 400 of FIGS. 3 and 4 are useful for certain applications, the present inventor has recognized a need for new approaches to suppressing diode reverse recovery current in switching power converters.